1. Field of the Invention
The present invention relates to an optical head device and an optical information recording/reproducing device for recording/reproducing information on an optical recording medium and particularly, to an optical head device and an optical information recording/reproducing device which can correct spherical aberration due to a deviation in the thickness of the substrate of the optical recording medium and coma aberration due to a tilt of the optical recording medium by using a liquid crystal panel.
2. Description of the Related Art
The recording density of an optical information recording/reproducing device is in inverse proportion to the square of the diameter of a focused light spot which is formed on an optical recording medium by an optical head device. That is, as the diameter of the focused light spot is smaller, the recording density is increased. The diameter of the focused light spot is in inverse proportion to the numerical aperture of an objective lens in the optical head device. That is, as the numerical aperture of the objective lens is higher, the diameter of the focused light spot is reduced. On the other hand, when the thickness of the substrate of the optical recording medium is deviated from the design value thereof, the shape of the focused light spot is distorted due to spherical aberration caused by the deviation of the thickness of the substrate, so that the recording/reproducing characteristic is deteriorated. The spherical aberration is proportional to the fourth power of the numerical aperture of the objective lens, and thus the margin of the thickness deviation of the substrate of the optical recording medium to the recording/reproducing characteristic is narrower as the numerical aperture of the objective lens is increased.
Further, when the optical recording medium is tilted relatively to the objective lens, the shape of the focused light spot is distorted due to coma aberration caused by the tilt, so that the recording/reproducing characteristic is deteriorated. The coma aberration is proportional to the third power of the numerical aperture of the objective lens, and thus the margin of the tilt of the optical recording medium to the recording/reproducing characteristic is narrower as the numerical aperture of the objective lens is increased. Accordingly, for an optical head device and an optical information recording/reproducing device in which the numerical aperture of the objective lens is increased to enhance the recording density, it is necessary to correct the substrate-thickness deviation and tilt of the optical recording medium in order to prevent deterioration of the recording/reproducing characteristic.
An optical head device using a liquid crystal panel is known as an optical head device which can correct the substrate-thickness deviation and tilt of the optical recording medium. FIG. 14 shows the construction of a conventional optical head device which can correct the substrate-thickness deviation and tilt of the optical recording medium by using a liquid crystal panel.
The optical head device shown in FIG. 14 is the same type as described in Japanese Journal of Applied Physics, Part 1 of Vol. 38, No. 3B, pp. 1744–1749. A beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and incident as P-polarized light to a polarization beam splitter 3. Substantially 100% of the P-polarized light is transmitted through the polarization beam splitter 3, then transmitted through a liquid crystal panel 4v and a quarter wavelength plate 5 to be converted from linearly polarized light to circularly polarized light, and then the circularly-polarized light is focused onto a disc 7 by an objective lens 6. Light reflected from the disc 7 is transmitted through the objective lens 6 in the opposite direction, and then transmitted through the quarter wavelength plate 5 to be converted from the circularly polarized light to linearly polarized light whose polarization direction is perpendicular to that of the linearly polarized light on the going path. The linearly polarized light transmitted through the quarter wavelength plate 5 is transmitted through the liquid crystal panel 4v, and then incident as S-polarized light to the polarization beam splitter 3. Substantially 100% of the S-polarized light is reflected from the polarization beam splitter 3, transmitted through a cylindrical lens 8 and a lens 9 and then detected by a photodetector 10. The photodetector 10 is disposed at the intermediate position between the line foci of the cylindrical lens 8 and the lens 9.
FIG. 15 shows a pattern of the photodetection portion of the photodetector 10.
A light spot 19 corresponding to the light reflected from the disc 7 is detected by photodetection portions 18a to 18d achieved by dividing the photodetection portion of the photodetector 10 into four parts by a dividing line passing through the optical axis and extending in parallel to the radial direction of the disc 7 and a dividing line passing through the optical axis and extending in parallel to the tangent line direction of the disc 7.
Representing the outputs of the photodetection portions 18a to 18d by V18a to V18d respectively, a focus error signal can be achieved from the calculation of (V18a+V18d)−(V18b+V18c) on the basis of the astigmatism method. A track error signal can be also achieved from the calculation of (V18a+V18b)−(V18c+V18d) on the basis of the push-pull method. A reproduction signal recorded on the disc 7 is achieved from the calculation of (V18a+V18b+V18c+V18d).
FIG. 16A shows the construction of the liquid crystal panel 4v. 
The liquid crystal panel 4v has glass substrates 11x and 11y, and a liquid crystal portion 12n filled with liquid crystal molecules, which is sandwiched between the glass substrates 11x and 11y. A pattern electrode 13m is formed on one surface of the glass substrate 11x which confronts the liquid crystal portion 12n, and also an overall-surface electrode 14g is formed on one surface of the glass substrate 11y which confronts the liquid crystal portion 12n. The positions of the pattern electrode 13m and the overall-surface electrode 14g may be replaced by each other. The X-direction, the Y-direction and the Z-direction in FIGS. 16A to 16C correspond to the radial direction of the disc 7, the tangent line direction of the disc 7 and the optical axis direction, respectively.
FIG. 16B shows an electrode pattern formed for the pattern electrode 13m when the substrate-thickness deviation of the disc 7 is corrected by using the liquid crystal panel 4v. This electrode pattern 20a is divided into five areas 21a to 21e. The dotted line of FIG. 16B corresponds to the effective diameter of the objective lens 6. FIG. 16C shows an electrode pattern formed for the pattern electrode 13m when the tilt in the radial direction of the disc 7 is corrected by using the liquid crystal panel 4v. This electrode pattern 20b is divided into five areas 21f to 21j. The dotted line of FIG. 16C corresponds to the effective diameter of the objective lens 6.
Next, a method of correcting the substrate-thickness deviation of the disc 7 will be described with reference to FIG. 17.
The correction of the substrate-thickness deviation is performed by using the liquid crystal panel 4v having the pattern electrode 13m on which the electrode pattern 20a shown in FIG. 16B is formed. FIG. 17A shows a calculation example of the spherical aberration caused by the substrate-thickness deviation on the cross section in the radial direction of the disc 7. The abscissa represents the radius of the objective lens, and the ordinate represents wave aberration achieved by applying defocus to the spherical aberration. In FIG. 17A, a solid line represents wave aberration when the substrate-thickness deviation is not corrected, and it is represented by a biquadratic function of the radius of the objective lens 6. A dotted line represents wave aberration when the substrate-thickness deviation is corrected by using the liquid crystal panel 4v, and it is represented by the sum of the wave aberration before the correction and the wave aberration produced by the liquid crystal panel 4v. The wave aberration varies discontinuously at the boundaries of the areas 21a to 21e of FIG. 16B. It is apparent from FIG. 17A that the standard deviation of the wave aberration is reduced by using the liquid crystal panel 4v and the substrate-thickness deviation can be corrected.
FIG. 17B shows a calculation example of the wave aberration produced by the liquid crystal panel 4v on the cross section in the radial direction of the disc 7. The abscissa represents the radius of the objective lens 6, and the ordinate represents the correction amount of the wave aberration. The correction amount in each of the areas 21a and 21e of FIG. 16B is equal to zero, the correction amount in the areas 21b and 21d is represented by a, and the correction amount in the area 21c is represented by 2α (α represents a constant).
A method of correcting the tilt in the radial direction of the disc 7 will be described with reference to FIG. 18.
The correction of the tilt in the radial direction is performed by using the liquid crystal panel 4v having the pattern electrode 13m on which the electrode pattern 20b shown in FIG. 16C is formed. FIG. 18A shows a calculation example of the coma aberration on the cross section in the radial direction of the disc 7, which is caused by the tilt in the radial direction of the disc 7. The abscissa represents the radius of the objective lens 6, and the ordinate represents the wave aberration achieved by applying a lateral shift to the coma aberration. In FIG. 18A, a solid line represents the wave aberration when the tilt in the radial direction is not corrected, and it is represented by a tertiary function of the radius of the objective lens 6. A dotted line of FIG. 18A shows the wave aberration when the tilt in the radial direction is corrected by using the liquid crystal panel 4v, and it is represented by the sum of the wave aberration before the correction and the wave aberration produced by the liquid crystal panel 4v. The wave aberration varies discontinuously at the boundaries of the areas 21f to 21j of FIG. 16C. It is apparent from FIG. 18A that the standard deviation of the wave aberration is reduced by using the liquid crystal panel 4v, and thus the tilt in the radial direction can be corrected.
FIG. 18B shows a calculation example of the wave aberration on the cross section in the radial direction of the disc 7, which is produced by the liquid crystal panel 4v. The abscissa represents the radius of the objective lens 6, and the ordinate represents the correction amount of the wave aberration. The correction amount in each of the areas 21f and 21i of FIG. 16C is represented by −β, the correction amount in the area 21h is equal to zero, and the correction amount in each of the areas 21g, 21j is represented by β (β represents a constant).
Next, a method of driving the liquid crystal panel 4v will be described with reference to FIGS. 19A to 19C.
A constant voltage VCOM is applied to the overall-surface electrode 14g of FIG. 16A. The abscissa of FIGS. 19A to 19C represents the time, and the ordinate thereof represents the applied voltage. In FIGS. 19A to 19C, a solid line represents a voltage applied to first to third areas of the electrode pattern formed for the pattern electrode 13m of FIG. 16A, and it is represented by a rectangular wave having a frequency of about 1 kHz, which has VCOM at the center thereof and has the amplitude corresponding to V1 (V2, V3). At this time, the difference of the correction amount of the wave aberration between the first and second areas is proportional to (V1−V2), and the difference of the correction amount of the wave aberration between the second and third areas is proportional to (V2−V3).
In the case where the substrate-thickness deviation of the disc 7 is corrected, if the area 21c of FIG. 16B is set to the first area, the areas 21b and 21d are set to the second area, the areas 21a and 21e are set to third area, and V1−V2=V2−V3=Kα (K represents a proportionality coefficient), the difference of the correction amount of the wave aberration between the area 21c and the areas 21b, 21d and the difference of the correction amount of the wave aberration between the areas 21b, 21d and the areas 21a, 21e can be set to the same value α.
In the case where the tilt in the radial direction of the disc 7 is corrected, if the areas 21g and 21j of FIG. 16C are set to the first area, the area 21h is set to the second area, the areas 21f and 21i are set to the third area and V1−V2=V2−V3=Kβ (K represents a proportionality coefficient), the difference of the correction amount of the wave aberration between the areas 21g, 21j and the area 21h and the difference of the correction amount of the wave aberration between the area 21h and the areas 21f, 21i can be set to the same value .
FIG. 20 shows the construction of an optical information recording/reproducing device having the optical head device using the liquid crystal panel 4v. 
The optical information recording/reproducing device is constructed by adding a reproduction signal detecting circuit 16 and a liquid crystal panel driving circuit 17d to the optical head device shown in FIG. 14. The reproduction signal detecting circuit 16 detects a reproduction signal recorded on the disc 7 on the basis of the output of each photodetecting portion of the photodetector 10. The liquid crystal panel driving circuit 17d drives the liquid crystal panel 4v according to the driving method shown in FIG. 19 so that the amplitude of the reproduction signal is maximum, whereby the substrate-thickness deviation of the disc 7 and the tilt in the radial direction of the disc 7 can be corrected, thereby avoiding the harmful influence on the recording/reproducing characteristic.
The liquid crystal molecules of the liquid crystal portion 12n in FIG. 16A are nematic liquid crystal molecules. These liquid crystal molecules are oriented in the X direction of FIGS. 16A to 16C when V1 to V3 of FIGS. 19A to 19C are equal to zero, oriented in the Z direction when V1 to V3 of FIGS. 19A to 19C are sufficiently large, and oriented in the intermediate direction between the X and Z directions when V1 to V3 of FIGS. 19A to 19C are equal to the intermediate values. The liquid crystal molecules of the liquid crystal portion 12n have birefringence characteristic and thus the refractive indexes thereof to ordinary light and extraordinary light are represented by no, ne. Further, the refractive indexes thereof to the emission light from the semiconductor laser 1 and the reflection light from the disc 7 are represented by nf, nr.
The emission light from the semiconductor laser 1 is linearly polarized light parallel to the X direction of FIGS. 16A to 16C. Therefore, when V1 to V3 of FIGS. 19A to 19C are equal to zero, the light emitted from the liquid crystal panel 4v has only an extraordinary light component. On the other hand, when V1 to V3 are sufficiently large, the light emitted from the liquid crystal panel 4v has only an ordinary light component. When V1 to V3 are equal to the intermediate values, the light emitted from the liquid crystal panel 4v has both the ordinary light component and the extraordinary light component. Accordingly, nf is varied between ne and no in correspondence with the value of V1 to V3. At this time, representing the values of nf corresponding to V1 to V3 by nf1 to nf3 respectively, the difference of the correction amount of the wave aberration in the first and second areas of the electrode pattern formed for the pattern electrode 13m of FIG. 16A and the difference of the correction amount of the wave aberration in the second and third areas are represented by (nf1−nf2)h/λ, (nf2−nf3)h/λ respectively when they are standardized by the wavelength of the incident light. Here, h represents the thickness of the liquid crystal portion 12n, and λ represents the wavelength of the incident light.
That is, by driving the liquid crystal panel 4v according to the driving method of FIGS. 19A to 19C, the liquid crystal panel 4v produces wave aberration to light on the going path which corresponds to the emission light from the semiconductor laser 1. The reflection light from the disc 7 is linearly polarized light parallel to the Y direction of FIGS. 16A to 16C, so that the light from the liquid crystal panel 4v is ordinary light irrespective of the values of V1 to V3. Accordingly, nr is equal to no irrespective of the values of V1 to V3. At this time, the difference of the correction amount of the wave aberration in the first and second areas of the electrode pattern formed for the pattern electrode 13m of FIG. 16A and the difference of the correction amount of the wave aberration in the second and third areas are equal to zero. That is, even by driving the liquid crystal panel 4v according to the driving method shown in FIGS. 19A to 19C, the liquid crystal panel 4v does not produce the wave aberration to light on the return path which corresponds to the reflection light from the disc 7.
When the substrate-thickness deviation of the disc 7 is corrected in the conventional optical head device, if (nf1−nf2)h/λ=(nf2−nf3)h/λ=α, the wave aberration due to the substrate-thickness deviation and the wave aberration produced by the liquid crystal panel 4v are canceled by each other for the light traveling on the going path. However, the wave aberration due to the substrate-thickness deviation remains for the light travelling on the returning path. Further, when the tilt in the radial direction of the disc 7 is corrected in the conventional optical head device, if (nf1−nf2)h/λ=(nf2−nf3)h/λ=β, the wave aberration due to the tilt in the radial direction and the wave aberration produced by the liquid crystal panel 4v are canceled by each other for the light traveling on the going path, however, the wave aberration due to the tilt in the radial direction remains for the light traveling on the returning path. The reproduction signal recorded on the disc 7 and the track error signal are dependent on the intensity distribution of the light travelling on the returning path, however, they are not dependent on the phase distribution of the light. Therefore, the reproduction signal and the track error signal are not affected even when the phase distribution is varied due to the remaining wave aberration. On the other hand, a focus error signal is affected when the phase distribution is varied due to the remaining wave aberration because it is dependent on the intensity distribution and phase distribution of the light travelling on the returning path.
FIGS. 21A to 21C are calculation examples of the focus error signal and the peak intensity of a focused spot on the disc 7. The abscissa of FIGS. 21A to 21C represents defocus. In FIGS. 21A to 21C, a solid line represents a focus error signal standardized with a sum signal, and a dotted line represents the peak intensity of the focused spot standardized by the maximum value.
FIG. 21A shows a calculation example when the disc 7 has no substrate-thickness deviation and no tilt in the radial direction. In the case of FIG. 21A, the defocus at which the focus error signal is equal to zero is coincident with the defocus at which the peak intensity is maximum, the focus error signal has no offset and the sensitivity (an inclination at the zero cross point) of the focus error signal is high.
On the other hand, FIG. 21B shows a calculation example when the substrate-thickness deviation of the disc 7 is corrected by using the liquid crystal panel 4v. In the case of FIG. 21B, the sensitivity of the focus error signal is high, however, the defocus at which the focus error signal is equal to zero is not coincident with the defocus at which the peak intensity is maximum, and the focus error signal has an offset. Therefore, if the focus servo is carried out so that the focus error signal is equal to zero, the peak intensity is reduced, and the recording/reproducing characteristic is deteriorated.
FIG. 21C shows a calculation example when the tilt in the radial direction of the disc 7 is corrected by using the liquid crystal panel 4v. In the case of FIG. 21C, the defocus at which the focus error signal is equal to zero is coincident with the defocus at which the peak intensity is maximum, and the focus error signal has no offset. However, the sensitivity of the focus error signal is lower than that of FIG. 21A. Therefore, if the focus servo is carried out so that the residual error of the focus error signal is equal to a predetermined value, the residual error of the defocus is larger than that of FIG. 21A, so that the recording/reproducing characteristic is deteriorated.